1. Field of the Invention
This invention relates to a method for removing transesterification catalyst from polyether polyols. More specifically but not by way of limitation, the present invention relates the use of MgSO.sub.4 to remove any alkaline catalyst residual in poly(tetramethylene ether) glycol (PTMEG).
2. Description of the Related Art
It is well known that in the preparation of polyether polyols, generally, and specifically the polymerization of THF and/or THF with comonomers in which acetic acid and acetic anhydride are used the intermediate products will contain acetate or other end groups which must be subsequently convert to the hydroxyl functionality prior to ultimate use. Therefore a transesterification process is frequently employed which uses methanol in the presence of strongly alkaline catalysts. Typical catalysts for this purpose are alkali metal hydroxides or alkoxides. Consequently catalyst is present in the product and it is generally accepted that even trace amounts have a very detrimental effect on the properties of polyurethanes and polyester elastomers which are major products made from polyether glycols. Thus it has been recognized in the art that when the hydrolysis or alcoholysis has ended, the basic catalyst added must be virtually completely removed from the resulting poly(tetramethylene ether) glycols in a simple manner and without loss of useful product.
Several methods for removing alkaline transesterification catalyst residuals from polyols are described in the literature. For example, U.S. Pat. No. 4,137,396 describes a process, which is useful for polyols in the molecular weight range of 300 to 26,000, using adsorbents, such as synthetic magnesium silicate, in combination with both a hydrocarbon solvent and water to remove the alkaline catalysts. The catalysts comprise hydroxides and alkoxides of Li, Na, K, Rb and Cs. The normal concentrations of catalyst in commercially prepared polyols is described as being in the range of 1700 to 4000 ppm and it is said to be desirable to reduce this to a level of about 5 ppm or less. However no mention is made in the patent of actual catalyst levels after conducting the process. The process also uses solvents to recover polyols from the adsorbents and the solvents have subsequently to be removed from the polyols.
U.S. Pat. No. 4,985,551 describes a process for purification of polyols in the molecular weight range of 500 to 10,000 using a strongly acidic, macroporous cation exchange resin to remove alkaline catalysts. The catalysts comprise hydroxides and alkoxides of Li, Na, K, Rb and Cs. The use of an aqueous alcoholic solution of the polyols leads to greater mass transfer between the polyol and ion exchange resin bed. Alkali earth metal concentrations of less than 1 ppm are reached at greater than 50% utilization of resin capacity.
U.S. Pat. No. 4,460,796 discloses a process for the purification of poly(tetramethylene ether) glycols which uses orthophosphoric acid to neutralize the basic catalyst followed by separation of the inorganic salts formed. The catalysts comprise alkali metal hydroxides or alkoxides and calcium oxide or calcium hydroxide. An essential feature of the process is the use of the correct ratio of equivalents of alkaline catalyst to orthophosphoric acid.
A process for the purification of polyether glycols containing alkaline catalyst is described in Japanese Kokoku Sho 62-20970. Alkaline catalyst such as sodium or potassium hydroxide or methoxide is neutralized by the addition of an acidic salt in the presence or absence of water followed by dehydration and filtration. The acidic salt used includes bisulfates, bisulfites, bicarbonates and hydrogen phosphates of sodium or lithium. The neutralization is conducted at 30.degree.-100.degree. C. and dehydration is conducted 100.degree.-140.degree. C. under reduced pressure.
U.S. Pat. No. 4,306,943 describes a process for the purification of polyether polyols containing alkaline catalyst by neutralizing with a mineral acid having a dissociation constant greater than 0.001 at 25.degree. C., adding the hydrate of a metal salt to promote crystal growth of the salt formed by neutralization, distilling off water and filtering. By promoting crystal growth the filtration of salt formed is facilitated.
U.S. Pat. No. 3,833,669 describe various historical approaches to catalyst removal employing acid neutralization, associated salt formation followed by precipitate removal and identifies respective problems with such acid/base neutralization techniques. Specifically, this reference identifies the necessity to neutralize exactly to the equivalence point, the tendency for formation of fine crystalline states and possible acid discoloration of the polyalkylene ethers. The reference also acknowledges the use of a volatile acid including gaseous CO.sub.2 for overcoming some of the inherent problems and then discloses and claims an improved CO.sub.2 neutralization technique performed in the presence of a basic magnesium salt (e.g., magnesium oxide, carbonate or hydroxide) preferably in large excess and a few percent by weight of water. In this process the alkali metal catalyst residue, upon CO.sub.2 neutralization is incorporated into the magnesium basic salt as an alkali-magnesium carbonate double salt which is then separated from the molten polyether polyol.